RIgght...there is only one isp in this area, so yeah I am stuck because I have no idea how to relay through another server

Just to give you some ideas. I run our company mail server on a machine at home and behind my domestic ADSL. I have no control over the reverse on this IP address, so I pay ~$100/yr for a virtual server elsewhere and run exim on that. My Zimbra server connects to the virtual server using an OpenVPN link, and relays all outbound mail through that VDS.

I also have the VDS set up as a backup MX for my main Zimbra server, but that is another whole kettle of fish and admin workload.

It's not hard to set up. Zimbra setup for relaying is in the Wiki and is a piece of cake. The VDS end was not difficult either. Pretty much a base Debian install with an extra config tweak to allow relaying from the VPN subnet. The VDS provider set up the reverse for me when they configured the machine, so it all "just works".

I don't recommend or endorse the way I run things but with only 5 users and a local BESX instance it works for us.